Before we dig into the specifics of the products we've tested for you here today, let's take a quick look at the anatomy of a typical Solid State Drive. Though you may dismiss an SSD to be nothing more than a bunch of memory chips plunked down on a PCB, in reality a Solid State Disk, like a standard hard drive, is a bit more complex. Below is a block diagram, courtesy of Mtron, that shows the topology of their SLC-based (Single-Level Cell) SSD.
SLC SSD Block Diagram - Courtesy: Mtron Technology
Mtron's MSP 7500 series of Solid State Disks are built with a proprietary multi-channel parallel array memory controller that affords the product its snappy sustained read/write specs of 130MB/sec and 120MB/sec respectively. As you can see, the design incorporates an ARM7 based control processor with associated SRAM cache, as well as an SDRAM buffer cache for the host interface, similar to a standard hard drive. This storage subsystem also provides the drive both static and dynamic wear-leveling, which ensures the use of the drive's memory evenly to extend the products usable life, as well as ECC and bad block management. Unfortunately, this, in addition to its SLC NAND flash memory, also adds considerable cost. The Mtron MSP 7500 Pro series drive is actually the most expensive in our round-up, currently at $779 for its 32GB capacity. Hence its "Pro" moniker, which calls attention to the fact that this drive is targeted for industrial or more mission-critical applications that require high reliability as well as performance.OCZ Core Series MLC-based SSD - Component Side PCB
On the other end of the spectrum, we have OCZ's Core Series SSD, which is an MLC-based (Multi-Level Cell) design and typical of most MLC-based SSDs. As you can see in the picture above, the drive has Samsung MLC flash chips, a few passive components and a single JMicron SATAII to Flash controller. It too offers a host interface for the flash array as well as wear-leveling technology and ECC (error correction) and bad block management. The primary differences between MLC and SLC-based SSD designs, are cost, density and life expectancy. For example, the Mtron SLC drive has a specified 140+ year write endurance at 50GB writes per day. Conversely, Super Talent specifies their MasterDrive MX MLC drive at 32 years write endurance for 50GB/day. More on this, next.
Of SLC and MLC -
As we mentioned earlier, there are currently two types of flash technologies that SSDs are built on, SLC and MLC. In short, you can think of the SLC type as higher quality with higher reliability but also lower density and a lot more expensive. SLC flash stores a single bit value per cell (either a 1 or 0), while MLC has four prommable states; 00 for fully programmed, 01 for partially programmed, 10 for partially erased and 11 for fully erased. This quick table, courtesy of Super Talent, tells the story nicely, at least at the chip level.
As you can see, MLC flash offers 2 - 4 times the density of SLC but significantly lower endurance, by a factor of ten, in terms of its available number of write cycles. You'll also notice that SLC has a slightly faster read speed but the memory controller used in the SSD also plays a big factor. Probably the most significant difference at the chip level is SLC's block size, which is much more granular at half the size of MLC and it's part of the reason why small block erase/re-write speed with SLC flash is faster. In addition, SLC type SSDs generally incorporate DRAM and/or SRAM cache buffers, which also afford them some performance advantages in certain scenarios. However, all of this aside, it's safe to say that, due to its cost factor alone, SLC-based SSDs will remain as enterprise or industrial type products, while MLC-based SSDs will become pervasive for the mainstream consumer market.
Courtesy: SuperTalent Technology, Inc.
Now that you're armed with some background on flash SSD technologies, we'll get down to specifics with each of the products in our round-up today. We'll be looking at performance with two SLC-based drives, one from Mtron and one from OCZ, as well as two MLC-based drives from OCZ and Super Talent.